The present invention relates to cold forming tubular materials, and more particularly to forming structural members using liquid-filled metal tubes.
Tube hydroforming is a known method of cold forming metal tubes to create structural members, for example, for the automotive industry. In a typical hydroforming process, a tube is partially deformed by stamping it in a die. Then, internal hydraulic pressure exceeding the yield strength of the tube wall is applied to force the tube to expand and to conform to the die cavity--much like blowing up a balloon. Several references discuss hydroforming methods. These references include U.S. Pat. No. 5,339,667 issued Aug. 23, 1994 to Shah et al., entitled "Method for Pinch Free Tube Forming"; U.S. Pat. No. 5,070,717 issued Dec. 10, 1991 to Boyd et al., entitled "Method of Forming a Tubular Member with Flange"; and U.S. Pat. No. 4,744,237 issued May 17, 1988 to Cudini, entitled "Method of Forming Box-Like Frame Members"; and Sanjay Shah et al., Tube Hydroforming: Process Capability and Production Applications, Body Assembly & Manufacturing Proceedings, International Body Engineering Conference (Sept. 1994).
Hydroforming processes offer several advantages over conventional die-stamping processes for cold forming metal tubes. These advantages include reduced variation in the finished pieces, reduced number of steps needed to produce the finished pieces, improved structural integrity of the finished pieces, and eliminated need to join separately pressed parts by welding. However, hydroforming has the disadvantage of requiring expensive and specialized die machinery to handle the extreme pressures to which the tube must be exposed. In particular, hydroforming requires additional machinery external to the die, such as pumps and intensifiers, to boost the internal hydraulic pressure of the tube. Further, the high pressures required for hydroforming can be dangerous to machine operators.
Several variations of the hydroforming process exist. For example, U.S. Pat. No. 4,829,803 issued May 16, 1989 to Cudini, entitled "Method of Forming Box-Like Frame Members" discloses a step of hydraulically pressurizing the internal space of a tube prior to closing the die, to allow better control of the deformation of the tube wall during die closure. The pressure to which the tube is initially pressurized, typically about 300 p.s.i.g., is selected to be less than the yield limit of the tube wall, but high enough so that during die closure (i.e., stamping), as the upper and lower die sections compress the tube, the tube walls are forced evenly toward the corners of the die cavity. More specifically, as the die closes, the hydraulic pressure within the tube causes the tube wall to overcome the frictional forces tending to resist the tube wall's transverse slippage over the surface of the upper and lower die sections. Thus, the internal pressure is selected so that the tube wall slides over the surface of the die sections and avoids being pinched between the upper and lower die sections as they mate.
To assure that the internal tube pressure during the '803 process does not rise to cause yielding of the tube wall during die stamping, a pressure relief valve is positioned in one end of the tube, set to release the liquid at a pressure below the yield limit of the tube. However, since the tube wall at the completion of this stamping process is bowed or dished inwardly, the '803 process requires a final hydroforming step of applying internal pressure to exceed the yield limit of the tube wall, and to expand the tube to conform to the die cavity. Thus, the '803 process does not escape the disadvantages of the hydroforming process. Rather, the '803 process adds an initial pressurization step to the hydroforming process, thereby slowing the tube forming process and increasing the cost of hydroforming.
Another variant of the tube hydroforming process is described in U.S. Pat. No. 5,353,618 issued Oct. 11, 1994 to Roper et al., entitled "Apparatus and Method for Forming a Tubular Frame Member," which discloses hydraulically pressurizing the interior of a tube to just below its burst pressure (yield strength) prior to bending and die stamping the tube, in order to ensure uniform, non-buckling deformation of the tube. A pressure relief valve and a hydraulic pressure source act in concert to maintain the internal pressure within the tube at just below the tube's burst pressure during the bending and stamping steps.
In utilizing the '618 process, if the cross-sectional perimeter of the preformed tube is, in some areas of the tube, less than the cross-sectional perimeter of the die cavity, then the tube must be expanded into the small radiused corners of the die cavity by subsequent hydroforming. However, if the cross-sectional perimeter of the die cavity is approximately equal to the cross-sectional perimeter of the preformed tube, then the tube will conform to the die cavity without subsequent hydroforming if the internal tube pressure prior to die stamping is near, yet less than, the internal burst pressure of the tube. (See Col. 18, lns. 7-33.)
The '618 process has several disadvantages. The requirement that the tube be internally pressurized prior to die stamping adds a step that increases the complexity of the tube forming process, and increases the amount of equipment needed to complete the process. Further, subjecting the tube to high pressures prior to stamping requires a step that slows the forming process and therefore increases the cost of tube forming. Also, pressurizing a tube prior to stamping it decreases the safety of the stamping operation. A final disadvantage of the '618 process is the limitation that the internal pressure of the tube during the die stamping step remain below the yield strength (i.e., burst pressure) of the tube wall. This limitation ultimately requires an additional hydroforming step to expand the tube if the cross-sectional perimeter of the die cavity is greater than the preformed tube circumference.
It is known in the unrelated field of pipe bending that increasing the internal hydrostatic pressure of a pipe can help to prevent buckling or wrinkling of the pipe wall when bending the pipe. See, for example, U.S. Pat. Nos. 3,105,537 issued Oct. 1, 1963 to Foster, entitled "Bending Pipe"; U.S. Pat. No. 567,518 issued Sep. 8, 1896 to Simmons, entitled "Mechanism for Bending Pipe"; and U.S. Pat. No. 203,842 issued May 21, 1878 to Leland, entitled "Method of Bending Plumbers' Traps." The elevated hydrostatic pressures, although lower than hydroforming pressures, suffer the same disadvantages noted above. Further, the express purpose of pipe bending is to maintain the same cross-sectional roundness following bending.